1. Field of the Invention
The present invention relates to a condensate demineralization, and especially to an apparatus and a method for use in purification (demineralization) of condensate in power plants.
2. Description of the Related Art
In a facility such as, for example, a pressurized water reactor nuclear power plant, because there is a need to constantly maintain the quality of water within a steam generator pure, condensate water flowing into the steam generator from a condenser is purified by a condensate demineralizer(s). A boiling water reactor nuclear power plant is also equipped with a condensate demineralizer(s) for purifying condensate because there is a need for constantly maintaining the quality of water pure.
The condensate demineralizer is provided in order to demineralize condensate water by removing, using ion exchange resins, metal impurities leached from materials of construction such as pipes and salt impurities resulting from leakage of sea water used as the cooling water of the condenser.
The condensate demineralizer usually consists of a plurality of demineralization columns filled with a mixture of anion exchange resin and cation exchange resin for processing the condensate water and an external regeneration system which is designed to regenerate (rejuvenate) the ion exchange resins which have been exhausted, that is, when a breakthrough point has been reached. The exhausted ion exchange resins are transferred from the demineralization column to the regeneration system outside the demineralization section. The ion exchange resins thus regenerated are returned to the demineralization section for reuse.
The ion exchange resins can generally be categorized into either gel type or porous type, such as macroporous (MP) and macroreticular (MR), according to the structural characteristics. In those applications in which ion exchange resins are regenerated more frequently, porous type ion exchange resins with a greater physical strength (resistant to osmotic shocks) are generally used in order to allow for the swelling and contraction of the ion exchange resins.
When, on the other hand, the regeneration is not frequently required, such as in a case for a condensate water demineralizer for a boiling water reactor nuclear power plant, gel type ion exchange resins with a greater ion exchange capacity are typically used.
In either case, the anion exchange resin to be mixed and used with the cation exchange resin is usually of the same type as the cation exchange resin in terms of the porosity.
In any way, the performance of the ion exchange resin decreases as it is used over the years, and thus, there is a need for replacing the ion exchange resins once every few years. Performance decrease over years of use may be due to slough of organic materials such as polystyrene sulphonate (hereinafter abbreviated to xe2x80x9cPSSxe2x80x9d) from the cation exchange resin under an oxidizing atmosphere. In particular, hydrazine is commonly added to condensate water of a pressurized water reactor nuclear power plant to prevent rust in the pipes. The hydrazine oxidizes and decomposes upon contact with air used for scrubbing or the like performed during the regeneration of the ion exchange resin, leading to generation of hydrogen peroxide. As a result, the cation exchange resin decomposes and PSS or the like is given off.
In a boiling water reactor nuclear power plant, no chemical is added to condensate water for preventing rust, and demineralized water as such is used. Thus, normally, ion exchange resins are not oxidized by an oxidizing agent in the condensate water demineralizer. However, during periodic checkups of the power plant, water within the nuclear reactor is radiation decomposed, thereby generating hydrogen peroxide. The water within the nuclear reactor is then passed through the condensate water demineralizer after the operation is restarted. Because of this, water containing hydrogen peroxide is supplied to the condensate water demineralizer at a boiling water reactor nuclear power plant, promoting decomposition of the cation exchange resin just as in the case with a pressurized water nuclear power plant.
Moreover, leachables generated by the decomposition of the cation exchange resin, including the PSS described above, attach themselves to the anion exchange resin and contribute to a reduction of reactivity of the anion exchange resin. When the reactivity of the anion exchange resin is reduced, its performance of the removal of anion impurities (such as Clxe2x88x92 or SOxe2x88x9224) contained in condensate water is reduced, water quality in the nuclear reactor (boiling water reactor nuclear power plant) or in the steam generator (pressurized water nuclear power plant) is reduced, and corrosion of construction materials is promoted.
Furthermore, as a result of the reduction in the reactivity of the anion exchange resin, leachables from the cation exchange resin flow into the condensate water without being captured by the anion exchange resin, thereby resulting in a deterioration of the treated water quality.
One object of the present invention is to provide condensate water demineralization which can be used for processing condensate water, without giving rise to any serious degradation of ion exchange capacities. This demineralization method is particularly suitable for treating condensate water commonly found in power plants including pressurized water and boiling water reactor nuclear power plants.
The present inventors, after years of research to achieve the above object, found that by using a combination of a specific gel type cation exchange resin and a specific porous type anion exchange resin, a stable performance can be maintained even when the ion exchange resins come in contact with water containing hydrogen peroxide.
Specifically, the present invention relates to condensate water demineralization wherein a mixed bed of a gel type cation exchange resin and a porous type anion exchange resin is used, the cation exchange resin having a moisture holding capacity of 41% or less or a crosslinkage of 12% or greater.